Monolithic semiconductor integrated circuit (IC) resistors are commonly created by diffusing an impurity into a semiconductor substrate to create a controlled region of a conductivity type opposite to that of the substrate. The resistor value is determined by the resistor shape and size, which are geometric factors, and by the depth of diffusion and impurity concentration, which are process factors. Typically, the resistor is contacted by etching holes in the oxide layer on the semiconductor substrate at points adjacent to the resistor ends. Then when metallization is applied to the semiconductor, it will be in ohmic contact with the resistor ends thus providing the required connections. Since the metallization connection is via an aperture in the oxide, the aperture location will also influence the value of the resistor. For example, the size of the aperture and the spacing between end apertures both will have an effect on resistance. These aperture factors are geometric and are normally well enough controlled to be regarded as secondary tolerance factors.
Conventional IC devices usually do not require precisely controlled resistor values but often require precise resistor ratios. In particular, matched resistors are often employed. In this case, the resistors to be matched are located side by side and created so that whatever geometric factors apply to one, apply to the other. Since the two resistors are manufactured simultaneously, manufacturing variables will apply equally to both resistors. Thus, resistor matching (and ratioing)to a high degree of precision is available in the IC fabrication art. However, the absolute resistor value cannot be reproduced to high tolerance.
Typically, resistors are simply elongated diffused regions of semiconductor. In order to reduce sensitivity to contact variables, the resistor end is enlarged and a relatively large area metallization employed. This means that the resistors body proper largely determines resistance, and contact variables are second order effects.
The IC fabrication art has developed to where relatively narrow high value resistors can be created using diffusion to a tolerance of about .+-.15%. However, resistors can be matched to within about 2% and ratioed to about 5% (for 5:1 ratio).
While IC devices can be mass produced in circuits that do not require precision resistors, many applications require much more stringent tolerances. For example, in an 8-bit digital to analog (D/A) converter one IC approach is to employ a resistor ladder having 256 resistor elements. The ladder is usually formed from an elongated diffused region that is provided with side extensions that form taps thereon. The resistor and tap portions are formed simultaneously in a single diffusion operation so that the taps are accurately located. Therefore, regardless of process variables, the separate resistors will all be of the same value even though the actual value could vary substantially. However, the resistor elements at each end of the ladder must be contacted and their values are subject to the various processing variables. In conventional D/A converters, the resistor at one end of the ladder is made equal to one half of the value of the ladder resistors. The resistor at the other end of the ladder is made to have 1.5 times the resistor value. This is done to shift the ladder taps by one half bit to minimize conversion errors. In any event, it can be seen that while the 254 resistors in the body can be precisely matched, the two end resistors are subjected to the conventional resistor tolerances of IC manufacturing. In D/A converters, this is not ordinarily good enough. Accordingly, in precision D/A converters some sort of trimming is commonly employed. One way to do this is to fabricate the resistor to be trimmed as a deposited film on top of the oxide on the IC. Then after the circuit fabrication is completed, the film resistor can be precision trimmed with a laser or other removal means to a precise value.
The trimming approach is undesirable from several standpoints. It is costly because it requires extra equipment and processing. Since the end resistors are not the same as the rest of the resistors, they behave differently. Also it has been found that after trimming, many film resistors tend to drift in value. Clearly it would be desirable to incorporate the end resistors into the ladder and to make contact to them in a reproducable manner.